1. Field of the Invention
The present invention relates to a seed crystal for manufacturing a single crystal for use in manufacturing a single crystal semiconductor by a CZ method, a method of manufacturing the seed crystal for manufacturing a single crystal and a method of manufacturing a single crystal using the seed crystal.
2. Description of the Related Art
Single crystals, for example, single crystal silicon, are usually produced using the following CZ method. A quartz crucible disposed in an apparatus for producing single crystals is filled with polycrystalline silicon. A heater disposed in the periphery of the quartz crucible heats and melts the polycrystalline silicon, to thereby prepare a melt. A seed crystal joined to a seed holder is dipped into the melt, and then the seed holder is raised while the seed holder and the quartz crucible are rotated in the same or opposite directions. In this manner, single crystal silicon is grown to have a predetermined diameter and length.
When the seed crystal is dipped in the melt, thermal stress is produced, thereby causing dislocation in the seed crystal. To remove the dislocation caused by the thermal stress, a Dash neck method is employed to form a neck portion having a diameter of 3-4 mm at a position below the seed crystal. Thus, the dislocation is relieved to the surface of the neck portion. After no dislocation has been confirmed, a shoulder portion is formed. Then, the seed crystal is enlarged to have a predetermined diameter, and then a process for forming a straight body portion is started.
In recent years, either the diameter of the seed crystal has been enlarged or the axial length has been elongated in order to efficiently manufacture semiconductor devices and improve a manufacturing yield. The results are an increase in weight of the single crystal has been enlarged, and the neck portion approaching its stress limit and possibly breaking. Thus, there is concern that this related method is not reliable. As a result, the single crystal cannot safely be grown. As a countermeasure, a variety of single crystal manufacturing methods, which do not use the Dash neck method, have been proposed. For example,
(1) A method of raising the single crystal disclosed in Japanese Patent Laid-Open No. 9-249486 is structured to interrupt downward movement of the seed crystal at a position immediately above the melt so that pre-heating is performed. Then, the speed at which the seed crystal is moved downwards is gradually reduced when the seed crystal is dipped in the melt.
(2) A seed crystal for raising a single crystal disclosed in Japanese Patent Laid-Open No. 9-235186 has a conical leading end to reduce the thermal capacity. Thus, the temperature can easily be raised to the temperature of the melt.
(3) A method of raising a single crystal disclosed in Japanese Patent Laid-Open No. 9-249485 is structured to control the speed at which the seed crystal dipped in the melt to be very slow speed of 0.05-2 mm/minute. Thus, a dislocation portion, produced when the seed crystal is dipped into the melt, is melted.
(4) Moreover, a seed crystal having a hollow portion and a seed crystal having a bored side surface are employed to prevent a propagation of dislocation when they are dipped into the melt. In addition, a drawing process is omitted so as to enlarge the diameter of the non-dislocation seed crystal.
(5) A method of growing a seed crystal disclosed in Japanese Patent Laid-Open No. 4-104988 uses a seed crystal having a conical leading end is first pre-heated by a heater, and then dipped in the melt.
FIG. 18 is a graph showing change in the temperature at the leading end of a seed crystal realized when the seed crystal is gradually moved to approach the melt. The temperature at the leading end of the seed crystal is gradually raised in substantially proportion to the distance from the surface of the melt. While the rise in seed crystal temperature occurring before contact with the surface of the melt is limited,-simultaneously with the contact with the melt, the temperature rapidly increases upon contact. The dislocation produced when the seed crystal is dipped into the melt depends on the difference (100.degree. C. or greater) in temperature between the seed crystal and the melt. Since this difference is 100.degree. C. or greater, even if the seed crystal is held immediately above the melt, its temperature cannot be satisfactorily raised. Thus, it is understood that non-dislocation dipping is very difficult.
FIG. 19 is a graph showing a change in temperature at the leading end of the seed crystal as time elapses in a state in which the leading end of the seed crystal is secured 2.7 mm above the surface of the melt. If the seed crystal located at a still position above the surface of the melt, the temperature of which is 1420.degree. C., is allowed to remain in position for a long time, the temperature thereof cannot be raised in proportion to elapsed time. The atmospheric pressure in the furnace is about tens of Torr and the temperature of the seed crystal cannot sufficiently be raised only by radiation from the melt. This is because of the thermal conductivities of argon gas and silicon (argon gas is 0.05 W/m.multidot.K and silicon is 22.08 W/m.multidot.K), despite the supply of argon gas.
Therefore, the conventional techniques have the following problems:
(1) Even if the seed crystal is allowed to stand still immediately above the melt, the temperature cannot be raised in proportion to the standstill time.
(2) Even if a seed crystal having a conical shaped leading end is employed, the success rate of non-dislocation dipping is unsatisfactorily low because of the difference in the temperature between the seed crystal and the melt.
(3) If the seed crystal encounters even very small dislocation when dipping in the melt is performed, the dislocation is continuously propagated upwards even though melting of the seed crystal is performed at a very low downward movement speed. Thus, elimination of the dislocation cannot easily be performed. Therefore, the success rate of non-dislocation dipping is very low.
A variety of suggestions have been made in the conventional methods to eliminate the difference in the temperature from that of the melt when the seed crystal is brought into contact with the melt or the dislocation appearing when the contact has been performed. As described above, a rate of being non-dislocation adaptable to the industrial production cannot easily be realized.
(4) The method of previously heating the seed crystal by using a heating means is a preferred method because of its directness. However, a method using, for example, a heater which is provided for the seed holder, results in the apparatus for manufacturing a single crystal being complicated.